This project concerns the role of the vertebrate kidney in excreting organic non-electrolytes, especially urea; the mechanisms involved in tubular transport; and the control of these mechanisms. The approach is comparitive, which means transport of multiple organic non-electrolytes in multiple species (ranging from sheep, dog, and rat to shark) and in multiple tissues (kidney, erythrocyte). The interaction of renal metabolsim and function, the role of hormonal control, and the effect of diet are primary questions under study in the sheep at present.